1. Field of the Invention
The invention relates to fiber optic sensors generally, and more specifically to fiber optic sensors employing pressure-induced periodic gratings.
2. Discussion of the Background
Optical fiber sensors are becoming more popular for a wide variety of applications. Optical fiber sensors offer several advantages over other types of sensors such as electronic and mechanical sensors. Optical fiber sensors are generally more rugged and have longer lifetimes than these other types of sensors, are immune from electromagnetic interference, can often be made much smaller than these other types of sensors, and offer multiplexing capabilities.
One type of optical fiber sensor known in the art is the grating-based fiber optic sensor. These types of sensors employ an optical grating comprising a series of refractive index perturbations spaced along an optical fiber. The spacing is generally fixed, but “chirped” gratings with varying spacing are also known in the art. The optical grating can be either of two types—short period gratings (also referred to as Bragg gratings) and long period gratings.
Short period gratings have a periodic spacing less than the wavelength of an operating light source, typically less than one micron. These gratings convert light traveling in the forward-propagating guided fundamental mode to the reverse-propagating fundamental mode; that is, light traveling in the forward direction in the core of the fiber is reflected backward into the core by the grating. The wavelength of the reflected light depends upon the spacing in the grating. Therefore, if the spacing is changed, such as by expansion of the fiber due to a temperature increase or by compression or stretching of the fiber due to mechanical forces, a corresponding shift in the wavelength of the reflected light will occur. By applying broadband light to the fiber and analyzing the spectrum of the light reflected by the grating (or, conversely, by analyzing the spectrum of the light that passes the grating), the change in grating spacing, and thus the corresponding change in temperature and/or mechanical force applied to the fiber) can be determined. Short period gratings with periodic spacings of less than one micron have been widely used as temperature and strain sensors.
In contrast to short period gratings, long period gratings have a spacing greater than the wavelength of the operating light source. Typical spacings are between 15 and 1500 microns. Unlike short period gratings which reflect light backward into the core of the fiber, long period gratings couple light from a forward propagating mode in the core to another mode not guided by the core. For example, U.S. Pat. No. 5,641,956 describes sensor arrangements involving long period gratings in which light is coupled from the forward propagating fundamental mode in the core to a mode guided by the cladding of the optical fiber, where it is attenuated due to the lossy nature of the cladding mode. Alternatively, light traveling in the forward propagating fundamental mode can be converted into a higher order forward propagating mode guided by the core and subsequently stripped out to provide a wavelength-dependent loss.
The wavelength of light for which coupling occurs in the long period gratings is dependent upon the spacing of the grating. Thus, by examining the spectrum of the light that continues to be guided by a core of a fiber after passing through a long period grating formed in the core, changes in the spacing of the grating corresponding to changes in temperature and/or mechanical forces can be detected and measured.
Long period gratings can be formed using photolithographic processes involving the exposure of a doped (to increase photosensitivity) optical fiber to ultraviolet radiation. An example of such a process is described in U.S. Pat. No. 5,757,540. The amount of change in the refractive index caused by such gratings is generally permanent. The amplitude of the attenuation resulting from such gratings generally varies little when pressure perturbations are applied to this grating. Additionally, any changes in the amplitude of the attenuation peaks depends on temperature, pressure and strain, and it is therefore difficult to use a single grating of this type to measure any of these if they are present at the same time.
U.S. Pat. No. 6,282,341 describes optical fiber filters employing long period gratings formed by arcing across the fiber, such as with a commercial fiber splicer, at periodic intervals and/or by periodically stressing the fiber such as by maintaining pressure on a plate with milled periodically spaced ridges against the fiber. This patent includes no description or suggestion of employing long period gratings formed in such a manner in a sensor.